One of the most significant problems in the field of present-day electrophotographic reproducing systems is that of background or residual potential in exposed or non-image areas of the photoconductor. As is known in the art, in the course of operation of an electrostatic reproducing machine, the surface of a photoconductor first is brought under the influence of a corona discharge system which applies a predetermined electrostatic charge over the surface. Next, the charged surface is exposed to an image of the original to cause the charge to leak off in exposed or non-image areas and to cause the charge to be retained in unexposed or image areas of the surface. The resultant latent electrostatic image is then subjected to the action of a developer which is made up of a carrier and suspended toner particles having a triboelectric charge of a polarity opposite to that of the charge on the photoconductor surface. Toner particles tend to adhere to those areas of the photoconductive surface which retain the charge thus to develop the image. In plain paper copying machines after the latent image has thus been developed, the image is transferred to a sheet of plain paper. In most instances, this is accomplished by means of a transfer corona system which is of the same polarity as that of the charging corona so that, when the sheet of paper passes between the transfer corona and the surface carrying the developed image, the toner particles are transferred from the photoconductive surface to the sheet of paper.
The problem of background potential arises from the fact that most photoconductive surfaces do not discharge completely in non-image areas when the machine in which they are installed is operated at a practicable rate of speed. Stated otherwise, if the photoconductive surface were subjected to the light image for a time sufficient to permit exposed or non-image areas to discharge fully, the machine would not operate at a speed sufficiently high for commercial use. Thus, in most machines, after the exposure step there remains in the exposed areas a residual charge. When the photoconductor is run through a developer system, toner particles tend to migrate toward the exposed areas under the influence of the residual charge so that the resultant copy is gray in non-image areas rather than being pure white as is desirable.
Various expedients have been suggested in the prior art for overcoming the problem of deposition of toner particles in background areas. One such arrangement is illustrated in our copending application, Ser. No. 479,659, filed June 17, 1974, for Automatic Development Electrode Bias Control System. In the arrangement shown in that application, a floating electrode insulated from ground assumes a potential which is equal to the average potential across the latent image. This floating electrode potential is used to control the operation of a biasing circuit which applies a biasing potential to a development electrode which potential is of the same polarity as that of the image and is of a magnitude sufficient to overcome the effect of the residual or background potential. While the arrangement shown in the Schaefer et al. application successfully overcomes the problem of toner deposition in background areas, it is relatively expensive for the result achieved thereby. That is to say, that while it is eminently suitable for use in a relatively sophisticated machine, the cost of which warrants its inclusion, it is not suitable for inclusion in a relatively inexpensive machine.
We have developed an electrophotographic liquid developing system which overcomes the problem of toner deposition in background areas. Our system is simpler in construction than are systems of the prior art intended to achieve the same purpose. It is considerably less expensive to produce than are systems of the prior art. It permits of cleaning of the developer electrode in a relatively simple manner. Its operation is not appreciably affected by changes in the average potential of the photoconductor.